2011 Siat 2 Stroke For Range Extender


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Technical paper on the subject of Direct Fuel Injection 2-stroke engine for range extender application

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2011 Siat 2 Stroke For Range Extender

  1. 1. The Small Gasoline DI 2-Stroke Engine: an Keynote Paper Presented inAdapted Range Extender for Electric Vehicles ? SIAT-2011 Pierre Duret Present Position: Director of the Center for “Engines & Utilization of Hydrocarbons” at the IFP School Educational Background: 1981: Graduated from the French Engineer School “Ecole Centrale de Paris” Job Profile & Experience: 1982 – 1987: Research Engineer at IFP responsible of the study and development of two- stroke engines with direct fuel injection 1987 – 1996: IFP “Two-Stroke Engines” Projects Leader, responsible of a research and development group working on several projects of design and development of low emissions high fuel economy two-stroke engines and gasoline controlled auto-ignition engines for world-wide customers. 1996 - 2001: Assistant Director “Engines & Energy” at IFP 2001- 2003: Deputy Director of IFP “Engines & Energy” Technology Business Unit Since September 2003, Director of the Center for “Engines and Utilization of Hydrocarbons” at the IFP School In parallel, since May 2005, Chairman of the “Powertrain” Committee of the French Society of Automotive Engineers R&D Recent Involvement Expert for French Public authorities and for the European Commission in internal combustion engines Co-ordinator of several EU Projects, Network of Excellence and International Consortium Projects driven by IFP 1
  2. 2. Symposium on International Automotive Technology 2011 Publications & Events More than 30 families of granted patents and more than 50 international publications on engines and powertrains for automotive and other applications Organiser and chairman of several International Congresses on Powertrains Six “Best paper” Awards including one at the SIAT’99 2
  3. 3. Symposium on International Automotive Technology 2011The Small Gasoline DI 2-Stroke Engine: an Adapted RangeExtender for Electric Vehicles ?Pierre DuretIFP School, Rueil-Malmaison, FranceABSTRACT vehicle range (compared to the pure electric vehicle range)The main purpose of this paper is to discuss the possibility with only a few litres of gasoline.of using a small gasoline direct injected twostroke engine asa range extender for electric vehicles. INTRODUCTIONIn the first part of the paper, the most recently available In early 90’s, high fuel economy on a 500 kg concept carresults from DI two-stroke engines produced outside with a 2-cylinder 500 cc DI 2-stroke of 24 kW has alreadyautomotive as well as the performances achieved in the past been demonstrated by the author [1-3] asshown in theof some advanced DI two-stroke automotive concepts will be Fig.1. Nevertheless, this project was not further developed,reviewed and compared with the required specifications for a in particular because gasoline DI (direct fuel injection)range extender application. From this technical constructive technology was not mature at this period where in additionreview, it then becomes clearly possible to point out the emissions regulations were less severe than today’s andadvantages and limitations in considering the use of such future standards.engine technology as a range extender of electric vehicles.Then a detailed simulation study of a small electricautomotive vehicle equipped with a range extender isundertaken and their results are presented. These calculationsare done for several vehicle specifications (especially interms of maximum performance when the vehicle operatesin range extender mode). Compared to its two-cylinderfour-stroke counterpart, it is expected that a DI two-strokewould have a smaller displacement, size and weight, a lowercost (significantly lower if a single-cylinder configurationis chosen), much better NVH characteristics (if a two-cylinder is chosen), easier and less expensive maintenanceand significantly higher fuel economy. In addition the lower Figure 1. Fuel Economy Achieved in the Early 90’s withmaximum in-cylinder pressure of the two-stroke would make a 24 kW DI 2-Stroke in a 500kg Concept Carit particularly adapted to be combined with a starter generatorin the range extender application. From the simulation, it ispossible to understand that the main issue that would have It is particularly interesting to see that the first Ultra Lowto be carefully considered is probably the control of NOx Cost Car (ULCC) introduced in the Indian market weightsemissions to avoid the use of a costly DeNOx aftertreatment. 600 kg and is equipped with a 2-cylinder 623 cc 4-stroke of almost the same power output (25 kW). And this vehicleFinally the detailed results of the simulation show however has been homologated with a fuel economy 23,5 km/l. Eventhat in the case of the range extender application, such if has been probably not obtained under the same conditionstarget can be achievable provided that the engine operation (same driving cycle), it seems to be not as good as whatcan be maintained in the ultra low NOx Controlled Auto- was obtained in the past with the DI 2-stroke.Ignition (CAI) combustion range. Beside the achievementof the Euro 6 NOx target, remarkably low level of average Beside this new interest for ultra low cost passenger car,CO2 emissions can be achieved with impressively increased there is also a recent trend in powertrain development towards progressive electrification. Among the variouslevel 3
  4. 4. Symposium on International Automotive Technology 2011of powertrain electrification, the Electric Vehicle (EV) is Taking into account these two considerations (results achievedthe extreme one. Even if a lot of car manufacturers became in the 90’s with a DI 2-stroke with a non mature technologymore and more involved in this direction during the last few & recent availability of well proven DI 2- stroke technologymonths, it is generally considered that, due to its drawbacks outside automotive) it seems natural to wonder if for the ultramainly linked to the electric energy storage system (batteries low cost car as well as for the range extender application, aare very expensive, very heavy and need a lot of space for small DI 2-stroke engine could be a well adapted engine intheir packaging in the vehicle), the purely electric vehicle will place of the more conventional 4-stroke engine technologybe limited for some specific applications. This could change widely used in automotive applications. The Ultra Low Costin case of drastic progresses from the batteries in terms Car (ULCC) application of the DI 2-stroke engine has alreadyof cost and energy storage. But during the transition, the been studied in details in a previous paper [5]. In this newsolution to increase the chance of acceptance of EV by the paper the discussion will be specially focused on the rangepublic in a large scale could be to keep a limited pure EV extender application.range (with therefore minimum battery cost) correspondingto most of the urban usages and to equip the vehicle with THE PRINCIPLE ADVANTAGES OFa lightweight range extender. Such range extender would THE 2-STROKE CYCLEallow to exceptionally multiply by several times the pureEV range without sacrifying the global CO2 emissions. And The 2-stroke engine is well known for its main followingvagain as for the Nano example, it is interesting to remind specific advantages resulting from the principle of the 2-ourselves that Citröen presented in 1998 at the Paris Auto stroke cycle [1]:Show an electric vehicle (based on a Saxo Citröen model) 1. Low friction losses: this is particularly true with pumpand equipped with a small direct injected gasoline 2-stroke crankcase configuration (roller bearings for crankshaft,engine as range extender. This innovative vehicle (vehicle rod and piston pin; no oil ring retainer; no valves trainmass 1050 kg; max speed 120 km/h) was presented with to drive, one driving cycle every revolution), no oila pure EV range of 80 km and an extended range up to pump to drive especially during cold start;340 km. The auxiliary power unit used was a prototypeDI 2-stroke engine technology, 2 cylinder opposite 200 cc, 2. Low pumping losses : the pumping work decreasesdelivering a power of 6,5 kW and directly coupled with a in absolute value (almost constant in relative value asstarter generator. The auxiliary power unit (thermal engine shown by Figure 3b) when the load decreases. It is the+ starter generator) was remarkably packaged with overall contrary in a SI 4-stroke (Fig. 3a)dimensions of vol. 30x30x25 cm & a mass of 20 kg. 3. Double combustion cycle frequency when compared toWith such small size, it was possible to implement this a 4-stroke engineauxiliary unit under the rear seat of the Saxo car. A schematicview of the whole powertrain of the car is presented in The advantages 1 and 2 result in significantly higher effectiveFig. 2. power for the same indicated power, especially at part load as shown by the Fig. 3a and 3b. This should give a potentially higher 2-stroke fuel economy than SI 4-stroke. The advantage 3 results in higher specific torque and power output but nevertheless lower than 2 times the power of an equivalent 4-stroke because all the expansion stroke is not useful for producing power (exhaust port opens during the last part of the expansion stroke). As a consequence, the size and weight of a 2-stroke can be much smaller. It Figure 2. The Citroën Saxo Dynavolt: an Electric also allows to have drastically better 2-stroke NVH (noise, Vehicle Concept Presented in 1998 with a Small DI vibration and harshness) characteristics as we will also see 2-Stroke Engine as Range Extender [4] later in this paper. However in a classical carburetted 2-stroke engine, theBut this very interesting project was not further investigated potential fuel economy advantages 1 and 2 are unfortunatelyafter the 1998 Paris Auto Show for two main reasons: firstly, masked by the main 2-stroke drawbacks:it was not the right period for electric vehicles (too much inadvance !) and again 2-stroke gasoline DI technology was 1. The short-circuiting of fuel directly to the atmospherenot yet mature ! (above 50 % of maximum engine load) solved by DI (Direct fuel Injection)In parallel, during the last decade the DI 2-stroke technologyhas been further developed outside automotive and successfully 2. The poor combustion or misfiring (below 50% ofapplied in production for marine outboards and 2-3 wheelers maximum engine load) solved by combined CAIengines. (Controlled Auto-Ignition) & DI 4
  5. 5. Symposium on International Automotive Technology 2011 Figure 3a. Distribution of Indicative Power in Figure 4. Negative Effect of Losses from Mixture Effective Power, Friction Losses and Pumping Losses Shortcircuiting and of Losses from Irregular Combustion Versus Engine Load in a 4-Stroke Engine on Specific Fuel Consumption of a Carburetted 2-Stroke Engine [6] 2. Its combination with CAI combustion (Controlled Auto Ignition) for NOx emissions control and improved combustion stability [3,11,12] with the AR (Activated Radicals) combustion as an example of production available technology [6,13,14,15]. THE KEY SUCCESS FEATURES OF THE DI 2-STROKE ENGINE FOR RANGE EXTENDERS OF ELECTRIC VEHICLES After this introduction, the section of this paper will be organized in four main sub sections discussing the four main Figure 3b. Distribution of Indicative Power in Effective issues that can be considered are key success features of Power, Friction Losses and Pumping Losses Versus the DI 2-stroke engine as a powertrain for range extenders: Engine Load in a 2-Stroke Engine Simple, lightweight and compact: DI + exhaust throttling CAI - NVH issues and low production cost: single-cylinder +These two different sources of unburned fuel and therefore balancing shaft or 2-cylinder without balancing shaftof poor efficiency are clearly illustrated in the Fig. 4 as a - Easy maintenance and high fuel economy for lowfunction of engine load. operating cost: 2-stroke principle advantages and downsizingThe technologies to solve these two drawbacks already existand have been successfully introduced in production several - DeNOx free emissions control: oxidation catalystyears ago outside of automotive: with fast cold start lighting and CAI combustion for aftertreatment free NOx emissions control1. The gasoline direct fuel injection for HC emissions control and best fuel economy with several examples of production available technologies: The 2-Stroke Engine: A Simple, Compact and Lighweight Powertrain for Range - Air assisted direct fuel injection on marine outboard engines, autorickshaw, 2-wheelers [7] Extender - IAPAC compressed air assisted fuel injection on This is a well-known advantage of the conventional 2-stroke marine outboard engines [8,9] engine versus 4-stroke. The following Fig. 5 showing the compact range extender DI two-stroke engine arrangement - Direct liquid fuel injection on marine outboard under the rear seat of the Saxo Dynavolt clearly illustrate engines [10] this advantage. 5
  6. 6. Symposium on International Automotive Technology 2011 The reduction of the cylinder unit displacement is nevertheless limited towards low values by the increase of losses and the decrease of efficiency. On the other side, the reduction of the number of cylinders is limited by turbo charging and NVH issues. A 2-cylinder 4-stroke presents only one combustion cycle every engine revolution. It needs a balancing shaft to improve its NVH characteristics. For cost reduction, the best would be to use a single-cylinder engine, but if the 4-stroke cycle is still used, it would mean only one combustion cycle every two engine revolutions and therefore unacceptable NVH behaviour for automotive application. To use the 2-stroke cycle would double the combustion frequency and then provide an interesting solution to NVH issues at minimum production cost. A 2-cylinder opposite 2-stroke engine configuration (as shown by the Citroën Saxo example) would be the best solution in terms of NVH issues for a range extender application. A single-cylinder 2-stroke engine is even possible for minimum cost with NVH performance equivalent at least to a 2-cylinder 4-stroke engine and even better as shown in a previous paper [5]. Low Production Cost Engine Characteristics The 2-stroke engine is also particularly interesting in termsFigure 5. The Minimum Size of the 200 cc DI 2-Stroke of production cost. Its lighter weight means less materialsRange Extender Engine Installed under the Rear Seat of and therefore less raw materials cost. It is a simpler engine the Saxo Dynavolt [4] with much less components: the complete 4-stroke valve train system is deleted, inIn the previous section, we already explained that the addition if a 2-cylinder 2-stroke is used, there is no need2-stroke configuration adapted for such range extender of balancing shaftapplication should include DI technology combined with CAI There is also a way of significant further production costcombustion. As shown by several previous studies, the most saving (without sacrifying the NVH behaviour as explainedcost effective solution for implementing the CAI combustion previously) if a single-cylinder 2-stroke is used in place ofin a gasoline two-stroke engine is to use an exhaust throttling a 2-cylinder 4-stroke:device [2,16]. This device allows to control the exhaust backpressure and consequently the upstream internal scavengingand stratification process between the fresh charge and the there is still no valve train (but a balancing shaft becomesresidual gases. To use an AR (Activated Radical) exhaust necessary as in 2-cylinder 4-stroke) the number of movingvalve [6,13] or a transfer throttling valve [17,18,19] could parts (pistons, rings, rods,...) is reduced (divided by two)be slightly more efficient solutions but at a rather significant the number of fixed parts (fuel supply and injectors, ignitionincremental cost not justified for a range extender application. system,....) is also similarly reduced some parts become simpler: intake and exhaust manifold, crankshaft,...The DI 2-Stroke Engine: A Low Production For all these reasons, the 2-stroke engine technology canCost Powertrain with Significant NVH be considered as probably the cheapest to produce whileAdvantages in parallel giving the best NVH characteristics. What can have a negative impact on the cost of a DI 2- stroke areNVH Issues and Low Production Cost mainly the direct injection system and the possible need forFor a range extender application, a small size small an expensive specific DeNOx aftertreatment. Concerning thedisplacement engine is required for both compactness cost of DI 2-stroke technology, the progress done during theand lightweight (as described before) and also for best last few years and its various applications outside automotiveefficiency. The reduction of the overall engine displacement show that it can be probably considered as slightly highercan be achieved by two different ways: the reduction of the but almost similar to the cost of 4-stroke port fuel injectioncylinder unit displacement and the reduction of the number technology. Concerning the NOx emissions control, weof cylinders. will also see in a following section that there are some possibilities to achieve it without specific aftertreatment. 6
  7. 7. Symposium on International Automotive Technology 2011This is a key issue to keep the 2-stroke inherently low reduces HC emissions to a level almost similar (or slightlyproduction cost. higher) than 4-stroke, NOx emissions are significantly lower due again to the principle of the 2-stroke cycle (oneThe DI 2-Stroke Engine: Easy Maintenance combustion every cycle with half the 4-stroke IMEP) and ofand High Fuel Economy For Low Operating the inherent internal EGR dilution.Cost There are some possibilities for further reduction to ultra low level at low load thanks to the CAI combustion, RawEasy and Lower Maintenance Cost emissions of CO are generally significantly lower (lean burn operation at part load)for the customer The following 2-stroke engine specificfeatures have to be considered by the customer as providing A significant amount of scavenging air is directly short-easier maintenance at a lower cost: circuited and lost in the exhaust which means that there is always an excess of O2 in the exhaust.the 2-stroke mechanics is the simplest one and therefore somelimited maintenance operations can in some cases be directly This has two main consequences:done by the user himself, as it is done for example in India, - The exhaust conditions are highly favourable forthere is no requirement of oil change as in a 4-stroke engine. providing high efficient HC and CO conversion byThe oil tank can be easily refilled by the user himself on a an oxidation catalystregular basis as it would be recommended by the manufacturer - To maintain a minimum cost, a conventional 3-wayThis is something which has to be positively considered for catalyst aftertreatment cannot be the solution for NOxa low cost automotive range extender application. reduction and therefore the raw emissions of NOx haveHigh fuel economy for low operating cost Several examples to be maintained very low in order to avoid complexof DI 2-stroke engines in production outside automotive show DeNOx aftertreatment in oxidizing conditionsthe 2-stroke versus 4-stroke higher fuel economy thanks tothe principle advantages of the 2-stroke cycle. To illustrate it, If we look now again to some examples of DI two-strokefive different examples of applications have been described engines, we can start first with the liquid direct fuel injectedin a previous paper [5]. : 2-stroke outboard [10]. What is remarkable with this engine is that it is the first (and only one) outboard engine that50 cc 3,5 kW single-cylinder scooter application in Europe received the Clean Air Excellence Award of the US EPA144 cc 6,6 KW single-cylinder 3-wheeler application in India ! Its raw emissions performances were compared with250 cc 20 kW single-cylinder DI AR 2-stroke (compared to other technologies Including Fuel Injected (EFI) 4-stroke400 cc 4-stroke) for large scooter application technology. Almost the same HC + NOx emissions were obtained with significantly better CO emissions.680 cc 37 kW 2-cylinder marine outboard application1230 cc 52 kW 3-cylinder DI CAI 2-stroke automotive CAI Combustion for Aftertreatment Free NOxprototype compared to 1360 cc 4-stroke Emissions ControlThis paper clearly show the benefits in terms of fuel economy The emissions specifications for future vehicles will require toof the DI 2-stroke versus the 4-stroke engine. And this benefit meet a level similar to Euro 6: with high efficient oxidationis increasing when the engine size is decreasing (due to the catalyst (close coupled metallic substrate) and fast lightingincremental effect of the lower friction losses). control strategy for HC and CO emissions control and with aftertreatment free NOx emissions control The strategy usedNox Aftertreatment Free Emissions Control: for this purpose is also already described in details in athe Main Issue Of DI 2-Stroke for Range recent paper related to the Ultra Low Cost Car application [5]. NOx emissions can be controlled by using the ultra lowExtender NOx CAI (Controlled Auto-Ignition) combustion.This section deals with the emissions of a DI Two-stroke We can conclude from this second main section of this paperengine and about their control. that a small DI 2-troke engine presents some specific features – simple, compact and lightweight, low production cost withOxidation Catalyst for HC and CO Emissions significant NVH advantages, easy maintenance and high fuelControl economy for low operating cost, NOx aftertreatment freeDI 2-stroke engines present different emissions profiles than emissions control – that make it particularly well adapted4-stroke engines. : as a powertrain for ultra low cost passenger car application or as a range extender for electric vehicle.HC emissions are generally higher (intake and exhaust opensimultaneously in the 2-stroke cycle) but DI drastically 7
  8. 8. Symposium on International Automotive Technology 2011THE DI 2-STROKE ENGINE EV The engine displacement is not fixed and will be determined (as it will be described in the next subsection) accordingRANGE EXTENDER APPLICATION to the thermal engine power required to meet the target ofIn this third main section of the paper, we will study more in maximum vehicle speed achievable in range extender modedetails the DI 2-stroke range extender application for electric only. To fix the engine displacement, we considered a specificvehicles. For this purpose we undertook a simulation study. power of 42 kW/l which is easily achievable in a small DIThe conditions of this simulation will be first introduced and 2-stroke engine at a rather moderate maximum engine speedthen the results will be presented and discussed. (4500 rpm) in order to minimize engine noise. The combustion system of the DI 2-stroke engine is chosenConditions of the Simulation Study to avoid stratified charge direct injection generally responsibleVehicle Specifications of higher NOx emissions. Indeed since the control of NOx emissions without after treatment is probably the mostWe chose for this study an urban type of EV (electric important key issue, we prefer to chose the ultra low NOxvehicle). Its specifications are summarized in the Table 1 CAI (Controlled Auto-Ignition) mode at part load and to keephere below. homogeneous charge when the engine load increases. For a low cost small two-stroke engine, the simplest solution to get Table 1. Electric Vehicle Specifications the CAI combustion mode will be to use an exhaust throttling control valve, the position of the valve being controlled by the engine management system as a function of the engine load (intake throttle position sensor) and the engine speed. Regarding the exhaust conditions, due to the inherent 2-stroke scavenging process, there will always be an excess of short- circuited air in the exhaust. A closed loop 3-way catalyst cannot therefore be used. This is the reason why raw emissions of NOx have to be sufficientlyIt can be seen in the Table that a small urban vehicle was low to meet the legislation without complex and costlychosen. We chose a vehicle mass of 580 kg (similar to the DeNOx aftertreatment system. We also consider that anTata Nano used in the ULCC study [5]) with two possible oxidation catalyst has to be used for the control of CO andpure electric range. A load of 75 kg corresponding to the HC emissions. The excess of short-circuited air in the exhaustdriver was added. In the case of a 60 km EV range, the gases allows the oxidation to be extremely efficient. In DIadditional battery mass used in the simulation is 51 kg for 2-stroke engine applications, a metallic type of oxidationsuch small and light vehicle. It is increased to 104 kg for catalyst is preferred in order to obtain catalyst lighting atthe 120 km electric range. low exhaust temperature.During all the following simulations we also considered that Finally for all the simulations, we used engine efficiencythe vehicle was equipped with advanced low friction tyres (BSFC and CO2 emissions) as well as raw emissions ofand that there is a permanent electric power consumption of NOx coming directly for the extensive IFP DI 2-stroke150 W (power required by the auxiliaries). engine data base build during the last 25 years of experience [1,2,3,17,18].Specifications of the thermal engine used as range extenderRegarding the thermal engine used as range extender, its Efficiency of the Starter Generator, of the Electricspecifications are described in the following Table 2. Motor and of the Battery The main simplified assumptions used in the simulation Table 2. Specifications of the DI 2-Stroke Engine used regarding the efficiency of the energy conversion components as Thermal Engine Range Extender are summarized in the Fig. 6. As shown by the figure, we assume an efficiency of 0,9 for the starter generator to produce electric power from the thermal engine power. The efficiency of the electric motor is also assumed to 0,9 in both directions, to produce power to the wheels or reversely to recover energy during braking. Concerning the battery, we also use a simplified average efficiency of 0,8 for the storage of electric energy (coming either from the generator coupled with the thermal engine or from the electric motor during braking energy recovery) and for its redelivery from the battery to the electric motor. 8
  9. 9. Symposium on International Automotive Technology 2011Obviously more precise and more optimized efficiency data cc, it means that the preferred configuration (for minimumcould be used compared to what is used in this study. cost) will be to use a single cylinder engine if a REX vehicleNevertheless, it is important to point out that the main top speed of no more than 110 km/h is targeted, which willpurpose of this study is too really show the potential of DI be the most probable case for such type of urban vehicle.2-stroke engine (especially to demonstrate the capability to Above such targeted RE vehicle top speed of 110 km/h ameet NOx emissions legislation without DeNOx and to show two cylinder engine would probably be necessary.the low CO2 emissions and range extension potentials). It isnot at this stage to predict an actual project of range extender. Figure 6. Schematic View of the Thermal Engine andStarter Generator Package, of the Electric Motor, of the Figure 7. DI 2-Stroke Engine Displacement VersusBattery and of the Energy Management System Including Targeted Vehicle Top Speed in Range Extender (RE) Corresponding Efficiencies ModeDimensioning of the DI 2-Stroke Thermal Instantaneous Power Required to DriveEngine the Vehicle and Distribution of the Corresponding Energy FluxesThe power required for the thermal engine range extender(REX) depends on the target for the maximum vehicle speed Instantaneous Power Required to Drive the Vehicleachievable in range extender mode only (which means with on the NEDC Cyclebattery almost empty or with a charge below a minimumacceptable level). We made calculations of the power required The simulation model used is based on Excel. It calculatesto drive the vehicle for different choices of top speed from the instantaneous power required by steps of 0,5 second (as60 to 120 km/h. These calculations were done with a road mentioned before, this instantaneous power include the 150slope of 3% in order to give some margin in the use of the W permanent electric power consumption). The Fig. 8 showsvehicle. From such calculations and taking into account the an example of calculation for a vehicle with a top speed ofdifferent efficiencies described in the previous subsection, 80 km/h in range extender mode. According to the previousit is then possible to calculate the corresponding engine figure, such vehicle is then equipped with a thermal enginepower. From this engine power, and considering a DI of 273 cc with a maximum power of 11,5 kW.2-stroke specific power of 42 kW/l, we can then obtain theengine displacement necessary versus the vehicle top speedtargeted. Such results are reported in the following Fig. 7.Two curves can be seen, each one corresponding to twodifferent EV range.The dotted line is slightly above the full line because with120 km EV range the vehicle is slightly heavier (+ 53 kg ofbattery) which explains the slightly higher engine isplacementrequired. Nevertheless, as it can be seen in the figure, thedifferences between the two curves are very low.From such figure, it can be seen that if a vehicle top speedof 60 km/h is targeted in REX mode only, a DI 2- strokeengine of about 170 cc is sufficient while an engine of600 cc is necessary for a vehicle targeted top speed of 120 Figure 8. Instantaneous Power Required to Drive thekm/h. If we consider that the largest unit displacement used Vehicle Versus Time During the NEDC Driving Cyclein small DI 2-stroke engine is generally no more than 500 (with 80 km/h Maximum Vehicle Speed) 9
  10. 10. Symposium on International Automotive Technology 2011Two curves are presented on this figure: the vehicle speed(scale on the left side of the figure) versus time and thecorresponding instantaneous power required to drive thevehicle (scale on the right side of the figure) along theNEDC cycle with maximum speed limited to 80 km/h. It canbe seen that such instantaneous power oscillate a lot beingmaximum during the accelerations, being very low duringthe vehicle stabilized speed (only the remaining 150 Wwhen the vehicle is stopped) and becoming negative duringdeceleration and braking.Distribution of the Energy Dluxes During NEDCDriving Operation Figure 10. Relative State of Charge of the BatteryIn this example the calculated average power required by During the NEDC Cycle in RE Modethe vehicle along all NEDC cycle from the generator is 1,94 (with 80 km/h maximum vehicle speed)kW, which means 2,15 kW delivered by the thermal engine(with 0,9 efficiency of the generator). This figure shows that the state of charge of the batteryThis average electric power supplied by the engine/generator globally increases during the urban part of the driving cyclepackage is plotted on the Fig. 9 (full line with constant (even if some limited decrease can be observed during eachvalue). This figure presents also the instantaneous power acceleration) and then decreases significantly during therequired by the vehicle (dotted line) and the power supplied stronger accelerations of the extra urban part of the cycle.by the battery. In this figure, the instantaneous power of the At the end, the battery state of charge is even slightly higherbattery is negative when the battery supplies electric power to than at the beginning because of the energy recovery duringthe electric motor and is positive when the battery is loaded the last deceleration and braking.by electricity coming either from the electric motor (duringbraking) or from the generator (when it supplies an excess Final Results: NOx Emissions in Rex Mode,of electric power). Average CO2 Emissions and Electric Vehicle Extended Range Relation between Thermal Engine Operating Load and NOx Emissions From the Fig. 7 we have seen that the thermal engine displacement can be defined. Then from the calculations of the cycle and the example given in Fig. 9, we can get the thermal engine average power out put necessary to perform the NEDC cycle in range extender mode. The Fig. 9 gives a thermal engine average power of 2,15 kW (before the generator) for a maximum vehicle speed of 80 km/h in Figure 9. Instantaneous Power Distribution between the REX mode. For such given power, we made emissions and Vehicle, the Battery and the Thermal Engine (after the efficiency/CO2 calculations for three different engine speeds: Generator) Versus Time During the NEDC Cycle in RE 4000, 2500 and 1500 rpm. The new Fig. 11 shows that Mode (with 80 km/h maximum vehicle speed) for a maximum speed of 80 km/h (which means a 273cc 11,5 kW DI 2- stroke engine) such average power can beBattery State of Charge During the NEDC Cycle obtained with a BMEP of 1,02 bar @ 4000 rpm, of 1,63 bar @2500 rpm and 2,73 bar@1500 rpm. This figure presentsin REX Mode the other engine BMEP versus the maximum vehicle speedIt is important to point out that to perform all the simulations in REX mode (which is directly correlated with the enginein range extender mode, the main assumption is that when displacement as shown in Fig. 7 and repeated in the rightthe NEDC cycle is operated in range extender mode, the axis of this figure).thermal engine power is chosen in order to be neutral in In the next Fig. 12, we plotted the calculated BMEP of theterms of battery state of charge & discharge. This is clearly Fig. 11 versus engine speed for the four limited vehicle topshown by the Fig. 10 (which still corresponds to the same speed of 60, 80, 100 and 120 km/h (which correspond toexample of 80 km/h limited vehicle top speed in REX mode). a respective thermal engine displacement of 171, 273, 415 10
  11. 11. Symposium on International Automotive Technology 2011and 606 cc). We also add in this figure the typical ultra low • Condition A: the on board electric energy storage isNOx CAI combustion range. This CAI range is what can be fully charged.expected with the combination of an exhaust throttling valvewith DI which is the simplest and cheapest way of getting • condition B: the on board electric energy storage is atCAI in a DI 2-stroke. The definition of this range is based its minimum state of charge. To reach this condition,on the IFP DI 2-stroke database [5]. the vehicle is run at 50 km/h until the thermal engine start and the vehicle is stopped.From this figure, it can be seen that engine speed of 2500rpm and 4000 rpm are fully inside the ultra low NOx CAI The measurement of emissions then start after a macerationcombustion range. On the contrary, the lower 1500 rpm is period. From our understanding of the legislation, it seemsoutside the range and we will see in the next subsection that, that the pollutant emissions limits will have to be met inas we could expect, NOx emissions will be much higher at both conditions. In the example of our study, our simulatedthis engine speed. vehicles have a pure EV range of either 60 or 120 km/h. It means that there are both able to perform the condition A of the NEDC cycle in pure EV, which means without any pollutant emissions. For the condition B, the vehicle has then to meet the Euro 6 legislation in REX mode. Regarding HC and CO, there are generally low in a DI 2-stroke engine and easily converted by an oxidation catalyst as already discussed before and demonstrated in several papers [5,7,18,19]. They have therefore not been estimated in this study considering that the main key issue will be the NOx without aftertreatment. The NOx emissions have been estimated based on the data available in the IFP DI 2-stroke engine data base. The results are reported in the following Fig. 13 and 14. Regarding the calculation of the CO2, the legislation proposes Figure 11. DI 2-Stroke Engine BMEP for 3 Different a method to calculate an average weighted value depending Speeds Versus Limited Vehicle top Speed in RE Mode on the EV range. The formula used in such method is: M = (De x M1 + Dav x M2) / (De + Dav) in which: M = average weighted mass emissions of CO2 in g/km M1 = mass emissions of CO2 in g/km in condition A M2 = mass emissions of CO2 in g/km in condition B De = range of the vehicle in pure electric mode (measured according to the Annexe 9 of [22]) Dav = 25 km (assumed average distance between two battery charges) In our study, we can consider that with an EV range of 60 or 120 km, our simulated vehicle can perform the NEDC Figure 12. DI 2-Stroke Engine Pperating BMEP in RE condition A in pure EV mode. This means that M1 = 0 and Mode During NEDC Driving Cycle for 4 Engine the formula becomes: Displacements (= 4 vehicle top speeds in REX mode) M = (25 x M2) / ( 25 + 60) = (25 x M2) / 85 in g/km for the simulated vehicle with a 60 km pure EV range andNOx Emissions Results in REX Mode and Average M = (25 x M2) / (25 + 120) = (25 x M2) / 145 in g/km forCO2 Emissions the simulated vehicle with a 120 km pure EV range.We studied the emissions legislation that will be applied Based on these two formulas, the average CO2 emissions offor plug in hybrid and for EV with range extender. The the 6O km EV range vehicle are plotted in Figure 13 and thelegislation [22] considers two conditions of operation of the average CO2 emissions of the 120 km EV range vehicle arevehicle: plotted in Fig. 17, both together with the NOx emissions in REX mode only. All the results are presented for a limited 11
  12. 12. Symposium on International Automotive Technology 2011vehicle top speed from 60 to 120 km/h in REX mode. Therefore to use the lowest engine speed allowing to be in CAI combustion mode, which means around 2500 rpmIt is interesting to see that both figures 13 and 14 show that as shown by this study, would provide the best trade off inwhen the DI 2-stroke engine runs at low engine speed such terms on average CO2 versus NOx emissions. This conclusionas 1500 rpm, its raw emissions of NOX are too high to meet is valid for each vehicle EV range and for each vehiclethe Euro 6 limit. On the contrary, with higher engine speeds limited maximum speed in REX mode. This confirms thatsuch as 2500 rpm and 4000 rpm, the Euro 6 limit can be there is a great flexibility in the choice of the displacementmet without NOx after treatment and with some margin. of the DI 2-stroke engine and therefore of the maximumThis is well correlated with the Fig. 12 and confirms that vehicle speed in REX mode. We can also see that the lowestthe DI 2-stroke range extender must preferably run in CAI CO2 (about 15 g/km with 60 km EV range and less than 10combustion at part load to take benefit of the ultra low NOx g/km with 120 km EV range) are obtained when the vehicleemissions of this combustion mode. top speed is limited to 60 km/h. With a more reasonableRegarding the average CO 2 emissions, the situation is 80 km/h limited vehicle speed, the CO2 emissions remainopposite. The lowest engine speed gives the best CO 2 nevertheless quite low with less than 20 g/km with 60 kmemissions. EV range and just above 10 g/km with 120 km EV range. DI 2-Stroke: an Efficient Solution to Extend the EV Range Finally to conclude this study, we calculated the extension of the EV range if the vehicle is equipped with a fuel tank of 10 litres of gasoline. The last Fig. 15 shows the results for the 60 km EV range vehicle versus the thermal engine displacement / limited vehicle top speed in REX mode. Figure 13. NOx Emissions (G/Km) in REX Mode Only and Corresponding Average CO2 Emissions for 3 Different Thermal Engine Operating Speeds Versus Limited Maximum Vehicle Speed in REX Mode with 60 Km Range in Pure EV Mode Figure 15. Extension of the Pure EV Range with Various DI 2-stroke range extender displacement / with various limited vehicle top speed Again it can be seen that with the smallest thermal engine displacement (the lowest top speed in REX mode) the vehicle range can be impressively increased (multiplied by 9) with only 10 litres of fuel. With a less limited vehicle speed in RE mode such as 80 km/h, the pure EV range of 60 km can even be extended up to more than 400 km with such low amount of fuel. CONCLUSION The main purpose of this paper is to review in details Figure 14. NOx Emissions (G/Km) in REX Mode Only the most recently available results from DI two-stroke and Corresponding Average CO2 Emissions for 3 engines recently produced outside automotive as well as the Different Thermal Engine Operating Speeds Versus performances achieved in the past of some advanced DI two- Limited Maximum Vehicle Speed in REX Mode with stroke automotive concepts, and to compare them with the 120 Km Range in Pure EV Mode required specifications for an ultra low cost car application as 12
  13. 13. Symposium on International Automotive Technology 2011well as for a range extender application. From the technical stroke powertrain. Finally, India, with its great expertise inconstructive review presented in the first two main sections high efficiency small engines as shown by the 2-stroke DIof the paper, it then becomes clearly possible to point out the commercialized in autorickshaw [21], could take a leadingadvantages and limitations in considering the use of such position in achieving such challenge.engine technology in an ultra low cost passenger car or as REFERENCESa range extender of electric vehicles. The following Table 3summarizes the main conclusions achieved. 1. Duret P, Ecomard A and Audinet M, “A New Two-Stroke Engine with Compressed Air Assisted Fuel Injection forCompared to its two-cylinder four-stroke counterpart, it High Efficiency Low Emissions Applications”, SAEis expected that a DI two-stroke would have a smaller Paper No. 880176, 1988displacement, size and weight, a lower cost (significantlylower if a single-cylinder configuration is chosen), much 2. Duret P and Moreau J F, “Reduction of Pollutantbetter NVH characteristics (if a two-cylinder is chosen), Emissions of the IAPAC Two-Stroke Engine witheasier and less expensive maintenance and significantly higher Compressed Air Assisted Fuel Injection”, SAE Paperfuel economy. In addition the lower maximum incylinder No. 900801, 1990pressure of the two-stroke would make it particularly adaptedto be combined with a simple stop and start system for 3. Duret P, “The Key Points for the Development of anfurther fuel savings in the ULC application or with a starter Automotive Spark Ignition Two-Stroke Engine”, IMECgenerator in the range extender application. The main issue 389/278, FISITA 925021, London 1992that would have to be carefully considered is probably 4. Auto Concept, December 1998 issuethe control of NOx emissions to avoid the use of a costlyDeNOx aftertreatment. 5. Duret P, “The New Generation of Gasoline DI 2- Stroke Engines: a Powertrain for Innovative Ultra Low Cost Passenger Cars ?”, Keynote paper, SIAT’09, Pune January 2009 6. Ishibashi Y and Tsushima Y, “A Trial for Stabilizing Combustion in Two-Stroke Engines at Part Throttle Operation, in Duret P, A New Generation of Two- Stroke Engines for the Future?”, IFP International Seminar, Rueil-Malmaison, Editions Technip, 1993 7. Bell G, Brewster S and Ahern S, ‘Beyond 3 Star Emission Capability for Outboard Engines’, SAE Paper Table 3. Summary of the Selection Criteria of the Most No. 2007-32-0052, 2007Adapted Small Engine Configuration for Ultra Low Cost 8. Duret P., Dabadie J-C. and Colliou T., “Application ofAssenger Cars or for Range Extender of Electric Vehicles IAPAC Fuel Injection for Low Emissions Small Two- Stroke Engines” SAE Paper No. 951795, 1995We have shown in the last main section of this paper thatin the case of the range extender application, such target 9. Venturi S, et al., “From Development to Industrializationcan be achievable provided that the engine operation can of an IAPAC Marine Outboard DI 2-Stroke Engine”,be maintained in the ultra low NOX CAI combustion range. SETC Conference, Pisa Italy, 2001Beside the achievement of the Euro 6 NOx target, remarkably 10. ‘The new Evinrude E-TEC outboards’, IAME44-1low level of average CO2 emissions can be obtained withimpressively increased vehicle range with only a few litres 11. Onishi S, et al., “Active Thermo-Atmosphere Combustionof gasoline. (ATAC) – A New Combustion Process for InternalFinally considering all these favourable conclusions and to Combustion Engines”, SAE Paper No. 790501, 1979answer to the question asked by the title of this paper, we 12. Duret P, “Two-Stroke CAI Engines” in Zhao H.,can conclude that it appears worthwhile to consider this “HCCI and CAI Engines for the Automotive Industry”,new generation of DI two-stroke engines as an attractive Woodhead Publishing Limited, 2007thermal engine for range extender of electric vehicles. Asingle-cylinder DI two-stroke gasoline engine able to operate 13. Ishibashi Y and Asai M, “Improving the Exhaustin controlled auto-ignition at part load installed as range Emissions of Two-Stroke Engine by Applying theextender in an EV or as powertrain in an ULC vehicle Activated Radical Combustion”, SAE Paper No. 960742,could probably be the best challenger along all the criteria 1996of Table 2 when compared to a more conventional four- 13
  14. 14. Symposium on International Automotive Technology 201114. Ishibashi Y, “Basic Understanding of Activated Radical CONTACT Combustion and its Two-Stroke Engine Application and Benefits”, SAE Paper No. 2000-01-1836 Pierre DURET e-mail address: pierre.duret@ifpenergiesnouvelles.fr15. Ishibashi Y, Nishida K and Asai M, “Activated Radical Combustion in High Speed High Power Pneumatic Direct Injection Two Stroke Engine”, in Duret P, A DEFINITIONS, ACRONYMS, New Generation of Engine Combustion Processes for the Future?, IFP International Seminar, Rueil-Malmaison, ABBREVIATIONS France, Editions Technip, 2001 AR : Activated Radicals (combustion)16. Tsuchiya K, et al, Emission Control of Two-Stroke CAI : Controlled Auto Ignition also named according to Motorcycle Engines by the Butterfly Exhaust Valve, the authors ATAC (Active Thermo Atmosphere SAE 800973 Combustion), HCCI (Homogeneous Charge Compression Ignition), AR (Activated Radicals)17. Duret P, Venturi S and Carey C, “The IAPAC Fluid Combustion,... Dynamically Controlled Automotive Two-Stroke Combustion Process” in Duret P, A New Generation of DI : Direct Injection (of fuel) Two-Stroke Engines for the Future ? Rueil-Malmaison, EPA : Environment Protection Agency France, Editions Technip 1993 EV : Electric vehicle18. Duret P and Venturi S, “Automotive Calibration of IAPAC : Injection Assistée Par Air Comprimé (which the IAPAC Fluid Dynamically Controlled Two-Stroke stands for “Compressed Air Assisted Fuel Combustion Process” SAE Paper No. 960363, 1996 Injection Technology”), trade mark of the IFP- developed DI 2-stroke technology19. Duret P et al., “The Air Assisted Direct Injection ELEVATE Automotive Engine Combustion System”, LPDFI : Low Pressure Direct Fuel Injection (brand name SAE Paper No. 2000-01-1899, 2000 used by the Selva Marine outboard company to market the IFP-developed IAPAC DI 2-stroke20. Barbusse S, “Motocycles, Cyclomoteurs; Energie et technology) Environnement”, Données et références – ADEME –June 2005 NVH : Noise Vibration and Harshness PDI-AR : Pneumatic Direct Injection with AR combustion21. Bajaj Press Release, Pune 8th December 2007 RE : Range extender22. Journal officiel de l’Union européenne – Règlement ULCC : Ultra Low Cost (passenger) Car No. 83ACKNOWLEDGMENTSThe author would like to particularly thank Thierry Colliouof the IFP Energies Nouvelles for the most recent enginesdata he provided and for the very useful calculations hemade and that were used in this study and Yoichi Ishibashiof Honda R&D for his precious advices and support, andfor some materials and results used in this paper. 14